Low temperature synthesis of nanowires in solution

ABSTRACT

Methods synthesizing nanowires in solution at low temperatures (e.g., about 400° C. or lower) are provided. In the present methods, the nanowires are synthesized by exposing nanowire precursors to metal nanocrystals in a nanowire growth solution comprising a solvent. The metal nanocrystals serve as seed particles that catalyze the growth of the semiconductor nanowires. The metal nanocrystals may be formed in situ in the growth solution from metal nanocrystal precursors. Alternatively, the nanowires may be pre-formed and added to the growth solution.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent application Ser. No. 11/582,132, filed Oct. 17, 2006, having the same inventors, now allowed, which is incorporated herein in its entirety, and which claims the benefit of U.S. Provisional Application No. 60/727,587, filed Oct. 17, 2005, having the same inventors, and which is incorporated herein in its entirety.

FIELD OF THE INVENTION

This invention generally relates to methods for producing nanowires in organic solvents at low temperatures.

BACKGROUND

Research in semiconductor nanowires has been fueled by their unique optical, electronic, and mechanical properties and their potential use in optoelectronic devices, chemical and biological sensors, computing devices, and photovoltaics (Law, M.; Goldberger, J.; Yang, P. D. Ann. Rev. Mater. Res. 2004, 34, 83). Semiconductor nanowires have been synthesized under a wide range of conditions, from high temperature (<1100° C.) gas-phase reactions to relatively low temperature (>250° C.) solution-phase conditions (Law, M.; Goldberger, J.; Yang, P. D. Ann. Rev. Mater. Res. 2004, 34, 83; Hu, J. T.; Odom, T. W.; Lieber, C. M. Accts. Chem. Res. 1999, 32, 435; Trentler, T. J.; Hickman, K. M.; Goel, S. C.; Viano, A. M.; Gibbons, P. C.; Buhro, W. E. Science 1995, 270, 1791; Yu, H.; Li, J. B.; Loomis, R. A.; Gibbons, P. C.; Wang, L. W.; Buhro, W. E. J. Am. Chem. Soc. 2003, 125, 16168; Fanfair, D. D.; Korgel, B. A. Crystal Growth & Design 2005, 5, 1971). Solution-phase routes to semiconductor nanowires are particularly interesting due to their potential for good size and shape control, chemical surface passivation, colloidal dispersibility, and high throughput continuous processes.

Nanocrystals and nanowires of Group IV materials, such as C, Si and Ge, have been extremely challenging to synthesize in solution due to their high crystallization energy barrier, and the propensity of those elements to form stable oligomeric species with hydrocarbons. In the past, crystalline Si and Ge nanowires and multiwall carbon nanotubes have been fabricated using supercritical solvents that are heated well above their boiling points to temperatures between 350° C. and 650° C. by pressurization (Holmes, J. D.; Johnston, K. P.; Doty, R. C.; Korgel, B. A. Science 2000, 287, 1471; Hanrath, T.; Korgel, B. A. J. Am. Chem. Soc. 2002, 124, 1424; Lee, D. C.; Mikulec, F. V.; Korgel, B. A. J. Am. Chem. Soc. 2004, 126, 4951). These temperatures exceed the decomposition temperatures of organosilane and organogermane precursors and the Au:Si and Au:Ge eutectic temperatures (˜360° C.), making it possible to promote Au nanocrystal-seeded nanowire growth (Shah, P. S.; Hanrath, T.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B 2004, 108, 9574). Although very high quality nanowires are obtained, it would be more desirable to synthesize crystalline nanowires under milder conditions to alleviate solvent decomposition and safety concerns.

SUMMARY

Methods for synthesizing nanowires in solution at low temperatures (e.g., about 400° C. or lower) are provided. In the present methods, the nanowires are synthesized by exposing nanowire precursors to metal nanocrystals in a nanowire growth solution comprising an organic solvent. The metal nanocrystals serve as seed particles that catalyze the growth of the semiconductor nanowires. The metal nanocrystals may be formed in situ in the growth solution from metal nanocrystal precursors. Alternatively, the nanocrystals may be pre-formed and added to the growth solution.

A broad range of semiconductor nanowires may be produced using the present methods. Non-limiting examples of such nanowires include nanowires composed of Group IV semiconductors, alloys of Group IV semiconductors, Group III-V semiconductors, and alloys of Group III-V semiconductors with other Group III and/or Group V elements, and Group II-VI semiconductors, and alloys of Group II-VI semiconductors with other Group II and/or Group VI elements. The nanowires may also be doped with magnetic impurities to form dilute magnetic semiconductors. Furthermore, the nanowires may have a core/shell configuration and/or may have a compositional modulation along their length.

Metal nanocrystals that may be used to seed nanowire growth include bismuth, tin, sodium, indium, lead and aluminum, with bismuth and tin being particularly preferred. The metal nanocrystals may be core/shell nanocrystals, such as gold, surrounded by bismuth, or gold, surrounded by lead. The metal nanocrystals may also be alloys, such as Au/Bi or Au/Pb.

For purposes of illustration only, a specific example of the types of nanowires that may be produced according to the present methods are germanium nanowires grown from bismuth seed nanocrystals using, for example, a germanium halide nanowire precursor. Another illustrative example is GaP nanowires grown from bismuth seed crystals using, for example, organophosphate and organogallium precursors.

Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the binary phase diagram of Ge—Bi and Ge nanowire growth mechanism.

FIG. 2(A-C) show SEM images of Ge nanowires made from GeI₂ in trioctylphosphine (TOP) at 350° C. with Bi nanocrystals as growth seeds (GeI₂:Bi=80:1 n:n). SEM reveals a high yield of high quality Ge nanowires with an average diameter of 50 nm. (C) A representative Ge nanowire shows a Bi nanoparticle at its tip. (D) Ge nanowires synthesized at a lower mole ratio of GeI₂ to Bi of 50:1 (n:n) were of lower quality.

FIG. 3 shows the EDS of a Ge nanowire: the black circles drawn in the TEM images in the insets indicate the beam position. (A) EDS of the nanowire tip shows both Ge and Bi with a ratio of ˜1:1 (Ge:Bi). The peaks of Cu and P are from the copper TEM grid and the solvent of TOP used in the reaction, respectively. (B) EDS taken from the nanowire core shows only Ge.

FIG. 4 shows high resolution TEM images of the Ge nanowires. (A) A 15 nm nanowire with <111> growth direction. (Inset) FFT indicates the nanowire was imaged down the [110] zone axis. The forbidden spot of [00 2] is from double diffraction of [ 1 1 1] and [1 1 1]. (B) Ge nanowire with a tip showing presence of Bi by EDS analysis. (Inset) A lower magnification TEM image of the wire with <111> growth direction.

FIG. 5 shows the XRD of Ge nanowire showing a diamond cubic crystal structure of Ge with {111}, {220} and {311} peaks. The peaks labeled with * are from hexagonal GeO₂.

FIG. 6 shows the HRTEM image of a Ge nanowire showing a <110> growth direction.

FIG. 7 shows organic monolayer-coated Bi nanocrystals used for SLS III-V nanowire synthesis. (A) TEM image of a field of Bi nanocrystals. (B) XRD from Bi nanocrystals, matching rhombohedral Bi (JCPDS Card 44-1246). Peak broadening and the Scherrer equation gives an average particle diameter of 20 nm. (Inset) High resolution TEM image of a Bi nanocrystal showing the internal crystallinity.

FIG. 8 shows (A-C) InAs and (D-F) GaP nanowires: (A,D) SEM and (B,C,E,F) TEM images. The FFTs in the insets in (B) and (E) index to sphalerite InAs and GaP, respectively, and indicate that the nanowires in (B) and (E) are imaged down the [011] zone axes. The nanowires exhibit a <111> growth direction. Both InAs and GaP nanowires were produced in trioctylphosphine/trioctylphosphine oxide (TOP/TOPO) in the presence of Bi nanocrystals.

FIG. 9 shows XRD from (A) InAs, (B) GaP, (C) GaAs, and (D) InP nanowires. Bi peaks are labeled with a (*). The peak labeled in (A) with (▪) is coesite (monoclinic SiO₂), the XRD substrate. InAs and GaAs nanowires were produced in trioctylamine (TOA). InP and GaP nanowires were produced in TOP/TOPO.

FIG. 10 shows (A) SEM and (B, C, D) TEM images of InP nanowires produced in TOP/TOPO in the presence of Bi nanocrystals. The inset in (A) displays an InP nanowire with the sphalerite crystal structure and <111> oriented growth. The (111) d spacing (3.388 Å) is labeled on the inset. (B) TEM image of an InP nanowire with stacking faults indicated by the ovals. (C) and (D) magnified images of the stacking faults indicated in (B).

FIG. 11 shows the (A) SEM image of GaAs nanowires produced using GaCl₃ in TOP/TOPO and the (B) SEM image of GaAs nanowires produced in TOA. The inset shows a TEM image of a GaAs nanowire with the sphalerite crystal structure and <111> oriented growth. The (111) d-spacing (3.263 Å) is labeled on the inset. Both syntheses were done in the presence of Bi nanocrystals.

FIG. 12 shows InAs nanowires synthesized using InCl₃ and (SiMe₃)₃As in (A) TOP/TOPO and (B) TOA.

FIG. 13 shows EDS data obtained from nanocrystals at the tip of (A) InAs, (C) GaP, (E) InP, and (G) GaAs nanowires compared to EDS from the (B) InAs, (D) GaP, (F) InP, and (H) GaAs nanowires. InAs, InP, and GaP nanowires were produced in TOP/TOPO. GaAs nanowires were produced in TOA.

DETAILED DESCRIPTION

The present invention provides methods for the low temperature synthesis of semiconductor nanowires in organic solvents.

The semiconductor nanowires are synthesized by exposing nanowire precursors to metal nanocrystals in a nanowire growth solution comprising an organic solvent using a solution-liquid-solid (SLS) growth process. In some cases, the solvent may be an ionic liquid. The metal nanocrystals serve as seed particles that catalyze the growth of the semiconductor nanowires. The metal nanocrystals may be formed in situ in the growth solution from metal nanocrystal precursors. Thus, in some embodiments, nanowire precursors and nanocrystal precursors are charged (e.g., injected), preferably simultaneously, into the growth solution which is held at an elevated temperature. However, in other embodiments, the nanocrystals may be added to the growth solution. The nanowires may be grown as free nanowires in the growth solution, or may be grown from a substrate surface. For example, the nanowires may be grown from nanocrystals that are tethered, or otherwise attached, to a substrate. Once the nanowire growth is sufficiently complete, the growth process may be stopped (e.g., by cooling the reaction mixture and/or diluting the solution with a hydrocarbon solvent) and the nanowires may be separated by one or more separation steps. An additional capping ligand may be added to the nanowire growth solution to coat the nanowires with an electrical and chemical passivation layer.

The methods are desirably carried out at an elevated temperature that is below the boiling point of the organic solvent. Because higher reaction temperatures generally lead to better nanowire crystallinity, high boiling point organic solvents are preferred. In some embodiments nanowire growth is carried out at a temperature of up to about 450° C. This includes embodiments where nanowire growth is carried out at a temperature of up to about 420° C., up to about 400° C., up to about 370° C., up to about 370° C., and up to about 350° C. For example, nanowire growth may be carried out at a temperature between about 200° C. and 350° C. This includes embodiments where nanowire growth is carried out at a temperature between about 300° C. and 340° C. The reactions may be carried out at ambient or elevated pressures. Where elevated pressures are used, however, the elevated pressures are not used to create supercritical conditions. Thus, the present methods are clearly distinguishable from known methods of producing semiconductor nanoparticles under supercritical conditions.

The nanowires produced in accordance with the present methods are characterized in that their length dimensions are significantly larger than their height or width dimensions. Typically the nanowires would have lengths of at least about 10 nm and height and width of no more than about 5 nm. In some instances the nanowires will have a length of at least about 100, 200 or even 500 nm. Longer nanowires may have correspondingly broader height and width dimensions. For example, in some instances, the nanowires may have heights and/or widths (i.e., diameters) of up to about 50 nm. In general, the nanowires will have aspect ratios of at least about 5. However, the nanowires produced in accordance with the present invention are not limited to those having such dimensions. In some cases the metal nanocrystal will remain at one end of the nanowires after nanowire synthesis.

Examples of semiconductor nanowires that may be synthesized by the present methods include, but are not limited to, Group IV semiconductor nanowires, Group III-V semiconductor nanowires, and Group II-VI semiconductor nanowires. Group IV nanowires include silicon (Si), germanium (Ge) and tin (Sn) nanowires. Group III-V nanowires include InAs, GaAs, GaP, GaSb, InP, InSb, GaN, InN, AlAs, AlP, and AlSb semiconductor nanowires. Group II-VI nanowires include cadmium-containing nanowires, such as CdS, CdSe, CdTe, and zinc-containing nanowires, such as ZnS, ZnSe and ZnTe.

The nanowires may also comprise alloys of Group IV elements or alloys of Group III-V semiconductors with other Group III or Group V elements (e.g., InGaAs or GaAsP nanowires). The use of alloy nanowires is advantageous because the alloy composition of the nanowires can be tailored to optimize the bandgaps for the nanowires depending upon their intended application. In some embodiments the alloy nanowires may have a composition that is modulated along their lengths. For example, the Group IV alloy nanowires may be produced with alternating bands of different Group IV elements (e.g., Ge/Si/Ge . . . ). Alternatively, the nanowires may be produced with a compositional gradient along their lengths. The compositional modulation may be accomplished by adding the appropriate precursor molecules at different times and/or at different concentrations during nanowire growth. The nanowires may also be core/shell type nanowires wherein the core nanowire has a different composition than the shell surrounding it. For example Group IV core/shell nanowires may have a core nanowire and a nanowire shell that comprises Group IV elements independently selected from the group consisting of C, Si, Ge and Sn.

Dopants may be introduced into the nanowires by introducing dopant precursors to the growth solution during nanowire growth. Suitable dopants include, but are not limited to, boron (B), phosphorus (P), arsenic (As), selenium (Se), rare earth metals, and lanthanides, such as europium (Eu) and samarium (Sm). Suitable dopant precursors include organometallics and salts or salt complexes thereof.

The nanowire precursors used to synthesize the semiconductor nanowires may be any precursor molecules that decompose in the growth solution to form semiconductor nanowires in the presence of the metal nanocrystals. The selection of appropriate nanowire precursors will depend, at least in part, on the desired composition of the nanowires, the nature of organic solvent and the reaction temperature.

In some embodiments of the present methods, the nanowire precursors are inorganic precursor molecules. The use of inorganic precursor molecules may be advantageous because they produce semiconductor nanowires with minimal organic contamination. In some cases, an additional reducing agent, catalyst, or free radical initiator may be added to the nanowire growth solution to promote precursor decomposition. However, organic nanowire precursors, such as organometallic precursor molecules may also be used.

Suitable inorganic precursors for Group IV semiconductor nanowires include, but are not limited to, Group IV metal halides. Specific examples of Group IV metal halides include, but are not limited to, GeI₂, GeI₄, GeCl₄, GeBr₄, SiI₄, SiCl₄, SiBr₄, SnI₄, SnCl₄, and SnBr₄.

Suitable organic precursors for Group IV semiconductor nanowires include organometallics. Organometallic precursor molecules include a Group IV metal and organic groups. Organometallic Group IV precursors include, but are not limited to organosilicon, organogermanium and organotin compounds. Some examples of Group IV precursors include, but are not limited to, alkylgermaniums, alkylsilanes, alkylstannanes, chlorosilanes, chlorogermaniums, chlorostannanes, aromatic silanes, aromatic germaniums and aromatic stannanes. Other examples of silicon precursors include, but are not limited to, disilane (Si₂H₆), trisilane (Si₃H₈), silicon tetrachloride (SiCl₄), trichlorosilane (HSiCl₃) and dichlorosilane (H₂SiCl₂). Still other suitable precursor molecules for use in forming crystalline silicon nanowires include alkyl and aromatic silanes, such as dimethylsilane (H₃C—SiH₂—CH₃), tetraethyl silane ((CH₃CH₂)₄Si) and diphenylsilane (Ph-SiH₂-Ph). In addition to germane, particular examples of germanium precursor molecules that may be used to form Ge nanowires include, but are not limited to, tetraethyl germane ((CH₃CH₂)₄Ge) and diphenylgermane (Ph-GeH₂-Ph).

To synthesize Group III-V nanowires at least one Group III element precursor and at least one Group V element precursor should be included in the growth solution. Examples of Group III and Group V precursors that may be used include, but are not limited to, tris(trimethylsilyl)arsine ((SiMe₃)₃As), indium chloride (InCl₃), gallium (III) acetylacetonate (Ga(acac)₃), gallium chloride (GaCl₃), tris(trimethylsilyl)phosphine ((SiMe₃)₃P), (^(t)Bu)₃Ga, and As(SiMe₃)₃. In some cases, the precursors may be single-source precursors containing a Group III element and a Group V element.

To synthesize Group II-VI nanowires at least one Group II element precursor and at least one Group VI element precursor should be included in the growth solution. Alternatively, a single-source precursor that includes a Group II element and a Group VI element may be used. Examples of Group II and Group VI precursors that may be used include, but are not limited to, cadmium oxide (CdO), zinc oxide (ZnO), diethylzinc (Et₂Zn), zinc acetate ((C₄H₆O₄)Zn), and solutions of trioctylphosphine-sulfide (TOP-S), trioctylphosphine-selenide (TOP-Se), and trioctylphosphine-telluride (TOP-Te).

In order to promote nanowire growth, various additives may be included in the growth solution. Such additives include reducing agents and free radical promoters. Superhydride, LiH and LiAlH are two examples of reducing agents that are well-suited for use with Group IV metal halide nanowire precursors. Diisobutylaluminum hydride may also be used.

In the present methods, metal nanocrystals provide seed particles for the nucleation and growth of the nanowires. Suitable metal nanocrystals include, but are not limited to, bismuth (Bi), tin (Sn), indium (In), lead (Pb) and aluminum (Al) nanocrystals. The nanocrystals may be made from a single metal, or may be composed of metal alloys. For example, any of the prior-mentioned metal nanocrystals could be alloyed with one or more of the following metals: gold (Au), silver (Ag), copper (Cu), nickel (Ni), cobalt (Co), iridium (Ir), manganese (Mn) and iron (Fe), to provide a metal alloy seed nanocrystal. Specific examples of alloy nanocrystals that may be employed as seed particles include Au/In, Au/Sn, Ag/Bi, Au/Pb, Ag/Ga, Ag/Sn and Ag/In nanocrystals.

The metal nanocrystals desirably have diameters that are no more than about 100 nm, more desirably, no more than about 50 nm, and still more desirably no more than about 10 nm. This includes nanocrystals having diameters of between about 1 and 50 nm, further includes nanocrystals having diameters of between about 1 and 10 nm and still further include nanocrystals having diameters of no more than about 5 nm. The nanocrystals may have a relative monodisperse or a polydisperse size distribution. For example, the size distribution of the nanocrystals may be relatively monodisperse with a standard deviation about the mean diameter of less than about 10%. Alternatively, the size distribution of the nanocrystals may be polydisperse with a standard deviation about the mean diameter greater than 10%, greater than 20%, or even greater than 25%.

The metal nanocrystals may be pre-formed and added to the growth solution. However, the nanocrystals are may also be formed in situ in the growth solution. This may be accomplished by charging metal nanocrystal precursors into the growth solution under conditions that promote the decomposition of the nanocrystal precursors and the nucleation of nanocrystal seed particles. Typically, the nanocrystal precursors are added to the growth solution along with one or more nanowire precursors. An example of a precursor that may be used to form Bi nanocrystals is Bi (III) 2-ethylhexanoate, which is desirably added to the growth solution along with a reducing agent, such as NaBH₄. Other metal nanocrystal precursors include metalorganics and metal carbonyls. Polyalcohols may be added in addition to these precursors to aid the formation of the seed metal particles in situ.

The solvents used in the nanowire growth solution are desirably carried out in organic solvents having high boiling points (e.g., at least about 200° C.). Examples of suitable organic solvents include, but are not limited to, phosphorus- or nitrogen-containing polar organic solvents, such as trioctylphosphine (TOP), trioctylphosphine oxide (TOPO), triphenylphosphine (TPP), triphenylphosphine oxide (TPPO), or an amine, such as trioctylamine (TOA). The use of these solvents is advantageous because they help to sterically stabilize the metal nanocrystals and/or provide nanocrystal size control as crystal growth terminators. Other suitable solvents include fluorinated solvents, ionic liquids (such as tetraoctylammonium bromide), ethers and coordinating solvents. Other non-limiting examples of suitable solvents include squalane and oleylamine. In addition to the solvent, the nanowire growth solution may include one or more capping ligands which coat the outer surfaces of the nanocrystals and/or nanowires. The capping ligands may act as a passivating layer and may help to tailor the nanocrystal/nanowire solubility characteristics. Examples of suitable capping ligands include, but are not limited to, thiols, phosphines, amines, alkenes, alkoxides and siloxanes.

The nanowires produced in accordance with the present methods are well-suited for use in a variety of applications including, but not limited to, electronic and optical devices such as optical sensors, optical detectors, photovoltaic cells, integrated circuit interconnects, field effect transistors, biochemical sensors, light-emitting diodes, and complementary logic devices.

EXAMPLES Example 1 Synthesis of Germanium Nanowires Using Bismuth Seed Nanocrystals

Bi Nanocrystal Synthesis.

Bi nanocrystals were prepared by arrested precipitation in dioctylether and TOP. 0.1 mL Bi(III) 2-ethylhexanoate (Bi[OOCCH(C₂H₅)C₄H₉]₃), and 0.15 mL TOP, was added to 11 mL dioctylether (C₁₆H₃₄O) and stirred for 15 min. Separately, the reducing agent (30 mg NaBH₄) was dissolved in 3.4 mL ethylenediamine by heating to 45° C. and stirring for 15 min. The reducing agent solution was cooled to room temperature and injected into the Bi precursor solution under vigorous stirring. The reaction mixture progressed from a milky appearance to a black color as the Bi precursor reduced to Bi metal nanocrystals. After 30 min, the crude nanocrystal dispersion was removed from the Schlenk line. The nanocrystals were precipitated with ˜5 mL ethanol and centrifuged at 8,000 rpm for 5 min. The supernatant was discarded. The dried nanocrystals were stored under nitrogen until needed, as the Bi nanocrystals were found to be very sensitive to oxidation and air exposure.

Ge Nanowire Synthesis.

Ge nanowires were synthesized under nitrogen in a three-neck flask on a Schlenk line. GeI₂ (Strem Chemicals, packed under argon) was stored in a nitrogen-filled glovebox upon purchase. Tri-n-octylphosphine (Tech. grade, Aldrich) was degassed through a freeze-pump-thaw technique and distilled at 200° C. and 0.3 Torr. All other solvents were used as received from Fisher Scientific without further purification. A precursor solution of 0.3 M GeI₂ in TOP was prepared by adding 0.2 g GeI₂ into 2 ml TOP in the glovebox. A clear yellow solution was formed after stirring for 1 hr. Bi nanocrystals were prepared following the procedures of Fanfair and Korgel, described above (Fanfair, D. D.; Korgel, B. A. Crystal Growth & Design 2005, 5, 1971). 0.6 mg Bi nanoparticles were mixed with 0.5 ml TOP and sonicated for 2 hrs to form a grey dispersion. 0.5 ml Bi nanoparticle dispersion and desired volume of GeI₂ solution in TOP (0.8 ml for GeI₂ to Bi mole ratio of 80:1, 0.5 ml for a ratio of 50:1) were loaded into a 3 ml plastic syringe. 4 ml TOP was loaded into a 3-neck flask and heated to 365° C. under N₂, followed by the injection of the GeI₂ and Bi nanocrystal solution through a rubber septum with stirring. The introduction of the precursor and Bi nanocrystals immediately produced a gray solution. The temperature dropped to 350° C. and the reaction was allowed to proceed at 350° C. for 10 min before taking the reaction flask off the heating mantle and allowing it to cool to room temperature. The nanowire dispersion was diluted with 10 mL toluene and centrifuged at 8,000 rpm for 5 min. The nanowires in the black precipitate were redispersed in 2.5 ml CHCl₃ and precipitated with 2.5 ml ethanol and centrifuged again to obtain a purified nanowire product. Nanowires were dried on a rotary evaporator and stored under nitrogen prior to characterization.

Materials Characterization.

The nanowires were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and powder X-ray diffraction (XRD). SEM samples were prepared by drop casting chloroform-dispersed nanowires onto Si substrates. SEM was performed on a LEO 1530 field emission gun SEM, operating at 10 kV accelerating voltage, and digital SEM images were acquired using an Inlens detector and LEO 32 software system. TEM specimens were prepared by drop casting dilute dispersions of nanowires after washing with 5% HCl acid for 5 min onto 200-mesh copper grids coated with lacy carbon film (Ladd Research Co.). High resolution TEM (HRTEM) was performed on a JEOL 2010F TEM; images were obtained digitally with a GATAN digital photography system. EDS was performed on the JEOL 2010F HRTEM equipped with an Oxford spectrometer. X-ray diffraction (XRD) was obtained from ˜2 mg nanowires deposited by drop casting on quartz slides (Gem Dugout, State College, Pa.) with continuous scan for 3 hours at a rate of 12°/min and 0.02° per step using a Bruker-Nonius D8 Advance powder diffractometer with Cu K_(α) radiation (λ=1.54 Å) and a rotary sample stage (30 rpm).

The low melting metal Bi, forms a eutectic with Ge at 271.4° C., which is well within the temperature window for conventional solvents. Based on the Bi:Ge phase diagram, Bi is a good candidate for Ge nanowire synthesis. Ge nanowire synthesis in a conventional solvent is also favored by a precursor that is sufficiently reactive at ˜350° C. One aspect of the present invention is based, at least in part, on the inventors' identification of GeI₂ as an effective precursor for Ge nanocrystal synthesis with high yields and minimal organic contamination. The synthesis of Ge nanocrystals using GeI₂ as a precursor is desirably carried out at 300° C. in TOP.

FIG. 1 outlines the germanium nanowire growth mechanism using GeI₂ as a precursor and TOP as an organic solvent. Two key aspects of the chemistry enable the production of crystalline Ge nanowires in TOP: (1) the low temperature Bi:Ge eutectic at ˜270° C. and (2) the highly reactive organic-free Ge precursor, GeI₂. GeI₂ disproportionates to Ge and GeI₄ with relatively high chemical yield at temperatures greater than ˜330° C.—far below its boiling point at 550° C. and the boiling point of many widely used solvents, e.g. Tri-n-octylphosphine (TOP). GeI₂ also forms a very soluble complex of (GeI₂).R₃P, when mixed with TOP, which dissociates at 120° C., making it an ideal precursor for SLS Ge nanowire synthesis. (King, R. B., Inorg. Chem. 1963, 2, 199.)

FIG. 2 shows Ge nanowires produced by injecting GeI₂ along with sterically-stabilized 20 nm diameter Bi nanocrystals into TOP at 365° C. on a Schlenk line, as described above. In this synthesis TOP served as a capping ligand that prevented significant nanocrystal aggregation but did not interfere with nanowire growth. The best Ge nanowire product was obtained using a GeI₂:Bi mole ratio of 80:1. Scanning electron microscopy (SEM) of the reaction product synthesized from GeI₂ and Bi nanoparticles in TOP at 350° C. showed a large amount of nanowires (FIG. 2). The wires were long and generally straight, ranging in diameter from 20 to 150 nm with an average of ˜50 nm, with aspect ratios exceeding 100. The average length of the nanowires is greater than 5 μm and many are longer than 10 μm. The product was nearly free of contaminants and the Ge nanowire reaction yield was very high: 7.5 mg of pure Ge nanowires were obtained in a typical reaction with 0.6 mg Bi nanoparticles and 75 mg GeI₂ in 5 ml TOP, corresponding to a yield of ˜40%. Bi particles could be observed at the ends of some Ge nanowires, as shown in FIG. 2C. Energy-dispersive X-ray spectroscopy (EDS) mapping of the observed confirmed that the particles are composed of Bi (FIG. 3). At a lower GeI₂:Bi ratio (GeI₂:Bi=50:1), both straight and tortuous wires were observed (FIG. 2D)—the tortuous wires generally have a similar average diameter but shorter length (<1 μm).

High resolution transmission electron microscopy (HRTEM) (FIG. 4) and X-ray diffraction (XRD) (FIG. 5) both confirmed that the nanowires were crystalline diamond cubic Ge. FIG. 4 shows two nanowires imaged by TEM: the nanowire growth direction was predominantly <111>. A small number of nanowires (˜10% of the sample) exhibited <110> growth direction (FIG. 6). A mixture of <111> and <110> growth direction is similar to what has been observed for Ge nanowires synthesized in supercritical solvents (Hanrath, T.; Korgel, B. A. J. Am. Chem. Soc. 2002, 124, 1424; Hanrath, T.; Korgel, B. A. Small 2005, 1, 717). The straight Ge nanowires are single crystals, nearly free of extended defects. The XRD pattern in FIG. 5 exhibits diffraction peaks corresponding to the {111}, {220}, {131} reflections of diamond cubic Ge. A small amount of GeO₂ can also be observed in the XRD pattern, which is consistent with TEM images that showed the nanowire surfaces to be oxidized.

Example 2 Synthesis of InAs, GaP, GaAs and InP Nanowires Using Bismuth Seed Nanocrystals

Chemicals.

All manipulations were done using Schlenk line techniques under nitrogen. Tris(trimethylsilyl)arsine ((SiMe₃)₃As) was prepared according to literature methods (McAlpine, M. C.; Friedman, R. S.; Lieber, C. M. Nano Lett. 2003, 3, 443-445). Indium (III) chloride (InCl₃), gallium (III) acetylacetonate (Ga(acac)₃), gallium (III) chloride (GaCl₃), tris(trimethylsilyl)phosphine ((SiMe₃)₃P), and bismuth (III) 2-ethylhexanoate were used as received from STREM. Ethylenediamine, dioctylether, sodium borohydride (NaBH₄), tri-n-octylphosphine (TOP), myristic acid, butanol, trioctylamine (TOA), and trioctylphosphine oxide (TOPO) were used as received from Sigma-Aldrich. All other solvents were used as received from Fisher Scientific without further purification.

Bi Nanocrystal Synthesis.

Bi nanocrystals were prepared using the methods described in Example 1, above. FIG. 7 shows TEM images and XRD data of the Bi nanocrystals used to synthesize the nanowires. The average nanocrystal diameter is ˜20 nm. The size distribution is relatively broad, however, there are very few particles much larger than 20 nm in diameter. The particles are spherical and crystalline and composed of Bi metal. The XRD pattern corresponds to rhombohedral Bi (JCPDS Card 44-1246). The peak broadening in the XRD data is consistent with nanocrystals with an average diameter of 20 nm based on the Scherrer equation.

Nanowire Synthetic Procedure.

Reagent solutions (detailed below) were mixed in a nitrogen filled glove box and injected into a hot (˜320-340° C.) coordinating solvent under nitrogen on a Schlenk line. Injection of the precursor solution decreased the solvent temperature by ˜40° C. Once the temperature increased back to the initial injection temperature, nanowire growth was allowed to proceed for 5 minutes before taking the reaction flask off the heating mantle and allowing it to cool to room temperature.

InAs Nanowires in TOP/TOPO.

A mixture of 61.4 mg InCl₃, 14.6 μL (SiMe₃)₃As, 5.8 mg Bi nanocrystals, 300 μL toluene, 24 μL oleic acid, and 850 μL TOP was injected into 1 g of hot (330° C.) TOPO. The reaction yield was ˜10%.

InAs Nanowires in TOA.

A mixture of 18 mg InCl₃, 8.6 μL (SiMe)₃As, 1.7 mg Bi nanocrystals, 300 μL toluene, 24 μL oleic acid, and 850 μL TOA was injected into a hot (340° C.) solution of 5.6 mg myristic acid in 2.5 mL TOA. The yield was ˜17%.

GaP Nanowires.

A mixture of 60.4 μL (SiMe₃)₃P, 1.7 mg Bi nanocrystals, 300 μL toluene, 24 μL oleic acid, and 850 μL TOP was injected into a hot (320° C.) solution of 76 mg Ga(acac)₃ in 1 g TOPO. The yield was ˜20%.

GaAs Nanowires.

A mixture of 8.6 μL (SiMe₃)₃As, 1.7 mg Bi nanocrystals, 300 μL toluene, 24 μL oleic acid, and 850 μL TOA was injected into a hot (340° C.) solution containing 14.3 mg GaCl₃, and 5.6 mg myristic acid dissolved in 2.5 mL TOA. The yield was ˜40%.

InP Nanowires.

A mixture of 18 mg InCl₃, 23.6 μL (SiMe₃)₃P, 1.7 mg Bi nanocrystals, 300 μL toluene, 24 μL oleic acid, and 850 μL TOP was injected into a hot (340° C.) solution of 2 g TOPO and 5.6 mg myristic acid. The yield was ˜57%.

Nanowire Purification.

After cooling to room temperature after the reaction, the crude nanowire dispersions were diluted with 10 mL toluene and centrifuged at 8,000 rpm for 10 min. The supernatant was discarded. The nanowires were redispersed in CHCl₃ and precipitated again with butanol or ethanol and recentrifuged to obtain a purified nanowire product in the precipitate. Nanowires were stored dry under nitrogen prior to characterization.

Materials Characterization.

Purified nanowires were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD). SEM samples were prepared by drop casting chloroform-dispersed nanowires onto glassy carbon substrates. SEM was performed on a LEO 1530 field emission gun SEM, operating at 3 kV accelerating voltage, and digital SEM images were acquired using an Miens detector and LEO 32 software system. For TEM, nanowires were drop cast from chloroform onto 200-mesh lacey carbon-coated Cu grids (Electron Microscopy Sciences). TEM and EDS were performed on a JEOL 2010F field emission gun electron microscope operating at 200 kV accelerating voltage. Energy dispersive X-ray spectra were obtained with an attached Oxford INCA spectrometer. Digital TEM images were acquired with a Gatan multipole scanning CCD camera. XRD data was acquired from ˜0.5 mg of nanowires on quartz slides using a Bruker-Nonius D8 Advance θ-2θ powder diffractometer with Cu K_(α) radiation (λ=1.5418 Å) and collecting with a scintillation detector for 6˜12 hours with an incremental angle of 0.02° at a scan rate of 12°/min.

InAs and GaP Nanowires.

FIG. 8 shows SEM and TEM images of InAs and GaP nanowires synthesized in TOP/TOPO with Bi nanocrystals. InAs nanowires were formed through the dehalosilylation reaction between InCl₃ and (SiMe₃)₃As, and GaP nanowires were synthesized by reacting Ga(acac)₃ with (SiMe₃)₃P. Nanowires did not form without Bi nanocrystals. Both InAs and GaP nanowires were obtained with relatively high yield and nanowires ranged between 1 μm and 10 μm long. The best nanowire reaction yields were observed with the addition of oleic acid and myristic acid. TOP is generally regarded as a relatively weak-binding capping ligand and the carboxylated ligands most likely provide additional Bi particle steric stabilization under the nanowire growth conditions.

XRD shows that the nanowires are crystalline and exhibit cubic (sphalerite) crystal structure (FIG. 9). Both InAs and GaP grow predominantly in the <111> direction, and FIG. 8 shows examples of

111

-oriented InAs and GaP nanowires. The lattice spacings in FIG. 8 match the (111) d-spacings of InAs (3.498 Å) and GaP (3.14 Å) and the angles between the [ 1 1 1] and [200] directions in the InAs (FIG. 8B) and GaP (FIG. 8E) nanowires are 55° and 56.3° respectively, which are characteristic of sphalerite (cubic) InAs and GaP.

InP Nanowires.

InP nanowires were synthesized in TOP/TOPO in the presence of Bi nanocrystals by the dehalosilylation reaction between InCl₃ and (SiMe₃)₃P. The nanowire yield was again relatively high and XRD shows that the InP nanowires are crystalline with sphalerite (cubic) structure (FIG. 9). FIG. 10 shows SEM and TEM images of an InP nanowire sample. The InP nanowires are crystalline with the <111> growth direction. However, the InP nanowires are generally not as straight as the InAs and GaP nanowires, and TEM images of the nanowire bends (FIGS. 10B-10D) reveal stacking faults as the possible source of bending. In comparison, SFLS-grown GaAs and GaP nanowires exhibit planar (111) twinning faults cross-sectioning the nanowires, unlike these stacking faults that appear at random orientations with respect to the growth direction (Davidson, F. M., III; Schricker, A. D.; Wiacek, R. J.; Korgel, B. A. Adv. Mater. 2004, 16, 646-649; Davidson, F. M., III; Wiacek, R.; Korgel, B. A. Chem. Mater. 2005, 17, 230-233). Although a very careful crystallographic analysis of the faults has not been carried out, they are most likely (111) stacking faults since the (111) planes are the slip planes in a sphalerite crystal (Takeuchi, S.; Suzuki, K. Phys. Stat. Sol. a 1999, 171, 99-103). Under poor growth conditions—such as starved In and P supply—stacking faults can be introduced into the nanowires to give rise to a tortuous morphology, as has been observed in Si nanowires (Lu, X.; Hanrath, T.; Johnston, K. P.; Korgel, B. A. Nano Lett. 2003, 3, 93-99).

GaAs Nanowires.

FIG. 11A shows SEM images of GaAs nanowires produced in TOP/TOPO from GaCl₃ and (SiMe₃)₃As. GaAs nanowires were also produced using alternative Ga precursors, including (tBu)₃Ga and Ga(acac)₃.

Trioctylamine, in place of TOP/TOPO as the solvent, was found to produce high quality GaAs nanowires in high yield from the dehalosilylation reaction with GaCl₃ and (SiMe₃)₃As. The phosphorus-containing TOP/TOPO mixture quenches GaAs nanowire formation. TOP/TOPO forms a relatively strong complex with Ga and complex formation competes with the dehalosilylation reaction. FIG. 11 compares GaAs nanowires produced in TOP/TOPO and TOA. The GaAs nanowires produced in TOA from GaCl₃ and (SiMe₃)₃As were crystalline with sphalerite crystal structure, as shown by the XRD pattern in FIG. 9. The GaAs nanowires grow in the <111> direction.

InAs nanowires could also be produced in TOA, rather than TOP/TOPO, although TOP does not quench the InAs formation reaction nearly as much as the GaAs reaction. FIG. 12 compares SEM images of InAs nanowires synthesized in TOA and TOP/TOPO. TOP influences the reaction significantly, but the In-TOP complex is not as chemically stable as the Ga-TOP complex and InAs nanowires form to a significant extent.

Bi Nanocrystals at the Tips of the Wires.

The XRD data in FIG. 9 for the GaP, GaAs, InP and InAs nanowires confirmed their sphalerite structure (InAs (JCPDS Card 15-0869), GaP (JCPDS Card 12-0191), GaAs (JCPDS Card 32-0389) and InP (JCPDS Card 32-0452)). In all of the nanowire XRD data, diffraction peaks also appear from Bi metal. The diffraction peaks are slightly narrower than those obtained from XRD of the starting Bi nanocrystals, indicating that Bi particle aggregation during the nanowire synthesis may occur to some extent. However, the peaks are still relatively broad, consistent with an average domain size in the 20˜40 nm range. This size range is consistent with TEM images that reveal 20˜40 nm diameter Bi particles residing at the tips of the nanowires, as shown in FIG. 13. EDS line scans (FIG. 13) confirmed that the particles are Bi, and are free of Ga and In metal, and that the nanowires are free of Bi.

Example 3 Synthesis of Silicon Nanowires Using Bismuth Seed Nanocrystals

Chemicals.

All manipulations were done using Schlenk line techniques under nitrogen. Trisilane (Si₃H₈) was used as received from Gelest. Bismuth (III) 2-ethylhexanoate, and tri-n-octylphosphine (97%, TOP), were used as received from STREM. Ethylenediamine, dioctylether, sodium borohydride (NaBH₄), trioctylamine (TOA), and squalane were used as received from Sigma-Aldrich. All other solvents were used as received from Fisher Scientific without further purification.

Bi Nanocrystal Synthesis.

Bi nanocrystals were prepared following the procedure detailed in Example 1.

Si Nanowire Synthesis.

3 ml squalane, which served as the nanowire growth solution, was added to a 4-neck reaction flask that was attached to a Schlenk line. The squalane was dried and degassed by heating the reaction flask to 100° C. under vacuum and then maintaining this temperature, while under vacuum, for 60 min. Separately, a reagent solution containing 1 ml TOA, 1.5 mg Bi nanocrystals, and 6 μl Si₃H₈ was prepared inside of a nitrogen filled glove box. Once the squalane was dried and degassed, the reaction flask was backfilled with nitrogen, and the temperature was increased to 416° C. The reagent solution was then rapidly injected into the hot (416° C.) squalane whereupon the temperature decreased by ˜40° C. Once the temperature returned to 416° C., nanowire growth was allowed to proceed for 5 min. before removing the heating mantle and allowing the reaction flask to cool to room temperature.

Nanowire Purification.

Once the reaction flask was cooled to room temperature, the crude nanowire dispersion was diluted with the addition of 10 ml of toluene. The nanowire product was purified by first centrifuging the crude nanowire dispersion, without adding any antisolvent, at 8,000 rpm for 10 min. The supernatant was discarded and the precipitate was redispersed in chloroform and precipitated once again by adding ethanol and recentrifuging.

Materials Characterization.

Purified nanowires were characterized by transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDS). For TEM, nanowires were drop cast from chloroform onto 200-mesh lacey carbon-coated Cu grids (Electron Microscopy Sciences). TEM and EDS were performed on a JEOL 2010F field emission gun electron microscope operating at 200 kV accelerating voltage. Energy dispersive X-ray spectra were obtained with an attached Oxford INCA spectrometer. Digital TEM images were acquired with a Gatan multipole scanning CCD camera.

Example 4 Synthesis of Se doped CdTe Nanowires Using Bismuth Seed Nanocrystals

Chemicals.

All manipulations were done using Schlenk line techniques under nitrogen. Trioctylphosphine oxide (99%, TOPO), tri-n-octylphosphine (97%, TOP), bismuth (III) 2-ethylhexanoate, and selenium powder were used as received from STREM. n-Octadecylphosphonic acid (ODPA) was used as received from Alfa Aesar. Cadmium oxide (CdO), ethylenediamine, dioctylether, tellurium powder, and sodium borohydride (NaBH₄) were used as received from Sigma-Aldrich. Solutions of tri-n-octylphosphine selenide (0.238M, TOP-Se), and tri-n-octylphosphine telluride (0.237M, TOP-Te) were prepared by dissolving 0.094 g elemental Se and 0.151 g elemental Te respectively in 5 ml TOP. All other solvents were used as received from Fisher Scientific without further purification.

Bi Nanocrystal Synthesis.

Bi nanocrystals were prepared following the procedure detailed in Example 1.

Se Doped CdTe Nanowire Synthesis.

A mixture consisting of 3 g TOPO, 0.018 g CdO, and 0.048 g ODPA was added to a 4-neck reaction flask that was attached to a Schlenk line. The TOPO/CdO/ODPA mixture was dried and degassed by heating the reaction flask to 100° C. under vacuum and then maintaining this temperature, while under vacuum, for 30 min. Separately, two reagent solutions were prepared inside of a nitrogen filled glove box. The first reagent solution consisted of 1.5 mg Bi nanocrystals, 121 μl TOP-Te, and 500 μl TOP. The second reagent solution consisted of 121 μl TOP-Se and 500 μl TOP. Once the TOPO/CdO/ODPA mixture was dried and degassed, the reaction flask was backfilled with nitrogen, and the temperature was increased to 340° C. The TOPO/CdO/ODPA mixture was stirred at 340° C. for 60 min. to enable the CdO to react with the ODPA. The formation of the Cd-ODPA complex is evidenced by the appearance of the TOPO/CdO/ODPA mixture which progressively changes from milky to clear. Once the hot (340° C.) TOPO/CdO/ODPA mixture became clear, the first reagent solution was rapidly injected into the reaction flask, whereupon the temperature decreased to 330° C. The second reagent solution was then rapidly injected into the reaction flask at 330° C., whereupon the temperature decreased to ˜320° C. After the addition of the second reagent solution, nanowire growth was allowed to proceed for 60 sec. before removing the heating mantle and allowing the reaction flask to cool to room temperature.

Nanowire Purification.

Once the reaction flask was cooled to room temperature, the crude nanowire dispersion was diluted with the addition of 10 ml of toluene. The nanowire product was purified by first centrifuging the crude nanowire dispersion, without adding any antisolvent, at 6,000 rpm for 5 min. The precipitate was discarded and the supernatant was precipitated by adding ˜20 ml methanol and centrifuging at 6,000 rpm for 8 min. The precipitate collected was the nanowire product.

Materials Characterization.

Purified nanowires were characterized by transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD). For TEM, nanowires were drop cast from chloroform onto 200-mesh lacey carbon-coated Cu grids (Electron Microscopy Sciences). TEM and EDS were performed on a JEOL 2010F field emission gun electron microscope operating at 200 kV accelerating voltage. Energy dispersive X-ray spectra were obtained with an attached Oxford INCA spectrometer. Digital TEM images were acquired with a Gatan multipole scanning CCD camera. XRD data was acquired from ˜0.5 mg of nanowires on quartz slides using a Bruker-Nonius D8 Advance θ-2θ powder diffractometer with Cu Kα radiation (λ=1.5418 Å) and collecting with a scintillation detector for 6˜12 hours with an incremental angle of 0.02° at a scan rate of 12°/min.

Example 5 Synthesis of ZnS, ZnSe, and ZnTe Nanowires Using Bismuth Seed Nanocrystals

Chemicals.

All manipulations were done using Schlenk line techniques under nitrogen. Trioctylphosphine oxide (99%, TOPO), tri-n-octylphosphine (97%, TOP), bismuth (III) 2-ethylhexanoate, and selenium powder were used as received from STREM. Zinc oxide (ZnO), diethylzinc solution (˜1M in hexane, Et₂Zn), oleic acid (60%, OA), oleylamine (70%), squalane, ethylenediamine, dioctylether, tellurium powder, sulfur powder, and sodium borohydride (NaBH₄) were used as received from Sigma-Aldrich. Solutions of tri-n-octylphosphine sulfide (1M, TOP-S), tri-n-octylphosphine selenide (1M, TOP-Se), and tri-n-octylphosphine telluride (0.237M, TOP-Te) were prepared by dissolving 0.16 g elemental S, 0.395 g elemental Se, and 0.151 g elemental Te respectively in 5 ml TOP. All other solvents were used as received from Fisher Scientific without further purification.

Bi Nanocrystal Synthesis.

Bi nanocrystals were prepared following the procedure detailed in Example 1.

Degassing Procedure.

After nanowire growth solutions (detailed below) were added to a reaction flask, they were dried and degassed by heating the reaction flask to 100° C. under vacuum and then maintaining this temperature, while under vacuum, for the times specified below.

Nanowire Synthesis Procedure.

Reagent solutions (detailed below) were mixed in a nitrogen filled glove box and injected into a hot nanowire growth solution (temperature indicated below) under nitrogen on a Schlenk line. Injection of the reagent solution decreases the nanowire growth solution temperature by ˜10° C. Once the temperature increased back to the initial injection temperature, nanowire growth was allowed to proceed for the time indicated below before taking the reaction flask off the heating mantle and allowing it to cool to room temperature.

ZnS Nanowire Synthesis.

A 4 neck reaction flask was attached to a Schlenk line and charged with 3 ml squalane, which served as the nanowire growth solution. The squalane was degassed for 60 min. The reaction flask was then backfilled with nitrogen and the temperature was increased to 350° C. A reagent solution consisting of 1.5 mg Bi nanocrystals, 144 μl Et₂Zn, 144 μl TOP-S, and 216 μl toluene was then rapidly injected into the hot (350° C.) squalane. Nanowire growth was allowed to proceed for 5 min.

ZnSe Nanowire Synthesis.

A 4 neck reaction flask was attached to a Schlenk line and charged with 3 g TOPO, which served as the nanowire growth solution, and 11.7 mg ZnO. The TOPO/ZnO mixture was dried and degassed for at least 60 min. The reaction flask was then backfilled with nitrogen, and 200 μl OA, which served as the Zn coordinating agent, was added to the dried and degassed TOPO/ZnO mixture. The TOPO/ZnO/OA mixture was then degassed for an additional 30 min. after which it was backfilled with nitrogen and heated to 350° C. The TOPO/ZnO/OA mixture was stirred at 350° C. for 20 min. to enable the ZnO to react with the OA. The formation of the Zn—OA complex is evidenced by the appearance of the TOPO/ZnO/OA mixture which progressively changes from milky to clear. Once the hot (350° C.) TOPO/ZnO/OA mixture became clear, a reagent solution consisting of 1.5 mg of Bi nanocrystals, 144 μl TOP-Se, and 360 μl toluene was rapidly injected into the reaction flask. Nanowire growth was allowed to proceed for 5 min.

ZnTe Nanowire Synthesis.

A 4 neck reaction flask was attached to a Schlenk line and charged with 3 ml oleylamine, which served as the nanowire growth solution. The oleylamine was degassed for 60 min. The reaction flask was then backfilled with nitrogen and the temperature was increased to 345° C. A reagent solution consisting of 1.5 mg Bi nanocrystals, 144 μl Et₂Zn, 144 μl TOP-S, and 610 μl TOP-Te was then rapidly injected into the hot (345° C.) oleylamine. Nanowire growth was allowed to proceed for 60 sec.

Nanowire Purification.

Once the reaction flask was cooled to room temperature, the crude nanowire dispersion was diluted with the addition of 10 ml of toluene. The nanowire product was purified by first centrifuging the crude nanowire dispersion, without adding any antisolvent, at 8,000 rpm for 10 min. The supernatant was discarded and the precipitate was redispersed in chloroform and precipitated once again by adding ethanol and recentrifuging.

Materials Characterization.

Purified nanowires were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD). SEM samples were prepared by drop casting chloroform-dispersed nanowires onto glassy carbon substrates. SEM was performed on a LEO 1530 field emission gun SEM, operating at 3 kV accelerating voltage, and digital SEM images were acquired using an Inlens detector and LEO 32 software system. For TEM, nanowires were drop cast from chloroform onto 200-mesh lacey carbon-coated Cu grids (Electron Microscopy Sciences). TEM and EDS were performed on a JEOL 2010F field emission gun electron microscope operating at 200 kV accelerating voltage. Energy dispersive X-ray spectra were obtained with an attached Oxford INCA spectrometer. Digital TEM images were acquired with a Gatan multipole scanning CCD camera. XRD data was acquired from ˜0.5 mg of nanowires on quartz slides using a Bruker-Nonius D8 Advance θ-2θ powder diffractometer with Cu Kα radiation (λ=1.5418 Å) and collecting with a scintillation detector for 6˜12 hours with an incremental angle of 0.02° at a scan rate of 12°/min.

For the purposes of this disclosure and unless otherwise specified, “a” or “an” means “one or more”. All patents, applications, references and publications cited herein are incorporated by reference in their entirety to the same extent as if they were individually incorporated by reference.

As will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges disclosed herein also encompass any and all possible subranges and combinations of subranges thereof. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as “up to,” “at least,” “greater than,” “less than,” and the like include the number recited and refer to ranges which can be subsequently broken down into subranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member.

While the principles of this invention have been described in connection with specific embodiments, it should be understood clearly that these descriptions are made only by way of example and are not intended to limit the scope of the invention. 

What is claimed is:
 1. A nanowire having a metal nanoparticle at one end thereof, wherein the metal nanoparticle is not attached to a substrate, wherein the metal nanoparticle comprises bismuth, and wherein the nanowire comprises (a) a Group IV element, (b) a Group III element and a Group V element, or (c) a Group II element other than cadmium and a Group VI element other than tellurium.
 2. The nanowire of claim 1, wherein the nanowire comprises a Group IV nanowire.
 3. The nanowire of claim 1, wherein the nanowire comprises a Group III-V nanowire.
 4. The nanowire of claim 1, wherein the nanowire comprises a ZnS nanowire or a ZnSe nanowire.
 5. The nanowire of claim 1, wherein the nanowire comprises at least one element selected from the group consisting of aluminum, nitrogen and tin.
 6. The nanowire of claim 1, wherein the nanowire is a Group IV semiconductor nanowire.
 7. The nanowire of claim 1, wherein the nanowire is a silicon nanowire.
 8. The nanowire of claim 1, wherein the nanowire is a germanium nanowire.
 9. The nanowire of claim 1, wherein the nanowire is a GaP nanowire.
 10. The nanowire of claim 1, wherein the nanowire is selected from the group consisting of GaN and InN nanowires.
 11. The nanowire of claim 1, wherein the nanowire is selected from the group consisting of GaSb nanowires, InSb nanowires, AlAs nanowires, AlP nanowires and AlSb nanowires.
 12. The nanowire of claim 1, wherein the metal nanoparticle is formed from a metal nanoparticle precursor, and wherein the metal nanoparticle precursor comprises an organometallic molecule comprising at least one element selected from the group consisting of Bi, In, Al, Pb or Sn.
 13. The nanowire of claim 1, wherein the nanowire comprises an alloy of two or more Group IV elements.
 14. The nanowire of claim 1, wherein the nanowire is a core/shell nanowire.
 15. The composition of claim 1, wherein the nanowire is doped with impurities.
 16. The nanowire of claim 1, wherein the nanowire comprises at least one element selected from the group consisting of Si and Ge.
 17. The nanowire of claim 1, wherein the metal nanoparticle comprises at least one element selected from the group consisting of Bi, In, Al and Pb.
 18. A composition comprising the nanowire of claim 1, wherein the nanowire is suspended in a liquid medium.
 19. A composition comprising a plurality of nanowires, wherein each of the plurality of nanowires has a metal nanoparticle which is disposed at one end which is not attached to a substrate, wherein the metal nanoparticle comprises bismuth, and wherein each of the plurality of nanowires comprises (a) a Group IV element, (b) a Group III element and a Group V element, or (c) a Group II element other than cadmium and a Group VI element other than tellurium.
 20. The composition of claim 19, wherein the nanowires are suspended in a liquid medium.
 21. The composition of claim 19, wherein the nanowire comprises at least one element selected from the group consisting of Si and Ge.
 22. The nanowire of claim 1, wherein the nanowire comprises InAs.
 23. The nanowire of claim 19, wherein the nanowire comprises InAs.
 24. The nanowire of claim 1, wherein the metal nanocrystals catalyze the growth of the semiconductor nanowires.
 25. The nanowire of claim 19, wherein the metal nanocrystals catalyze the growth of the semiconductor nanowires. 